Differential bus networks include one or more data buses connecting subcircuits of a system as a means of providing signal transmission to internal and external subcircuits incorporated within the system. Differential bus networks are typically used in electronic communication systems, such as, automotive multiplex wiring and computer interfaces. In particular, differential bus networks may include differential bus drivers applicable to voltage mode interface standards, such as RS422, Controller Area Network (CAN), Automotive Multiplex Wiring System (Abus) and Vehicle Area Network (VAN).
CAN systems are currently implemented as common networking systems for automotive and industrial applications. In particular, CAN systems provide a lightweight and cost effective means for the vehicle's central processing unit to communicate with satellite peripheral modules, e.g., dome lamps, door modules, headlight modules, taillight modules, anti-skid braking system (ABS) modules, airbag modules, etc. A typical arrangement includes a CAN wire having a one or two wire links routed throughout the entire vehicle. The CAN wire link is a twisted pair of two wires, CANH and CANL, which represent the high and low signals applied to the twisted pair, respectively.
In operation, a differential signal is applied to the CAN wire. This signal is transmitted and received on the opposite end of the CAN wire. When the two signals representing a high voltage and a low voltage, CANH and CANL, respectively, are the same voltage, the CAN wire is in a recessive state, wherein the recessive state indicates that no differential voltage exists across CAN nodes, CANH and CANL. When the two signals, CANH and CANL, have some differential in voltage, typically 2 volts, the CAN wire is in a dominant state, wherein the dominant states indicates that a differential voltage greater than 0.9 v differential exists.
The International Standards Organization (ISO) specifies that the CAN wire be at least ten meters in length. Unfortunately, at this length, the CAN wire acts as an ideal antenna which is subject to automotive-type high voltage transients, as well as industrial-type transients, such as radiated emissions, electromagnetic interference (EMI) and electrostatic discharge (ESD).
In general, radiated emissions interfere with various types of electronic equipment, such as cell phones and radios or any other electronic device. Although most electronics have radiated emissions to some degree, radiated emissions is of substantial concern in automotive systems, due to the variable amount of electronics in automobiles. Specifically, within a CAN system, radiated emissions are high frequency signals that are emitted from the CAN wire.
In order to operate in the harsh environments of automotive and industrial settings, a CAN transceiver must successfully withstand these high voltage transients and must be capable of handling the standard automotive requirements of double battery and a 40 volt load dump. It must also withstand shorts from the CAN wire to Vcc, ground, and any other power supply associated with the system. These requirements are typically specified as the ability to survive voltages on the CAN wire(s) between +40 and −27 volts.
Furthermore, besides the ISO length requirement being the primary cause of radiated emissions, radiated emissions occur when there is no symmetry between the CAN signals, CANH and CANL. If the CAN signals, CANH and CANL, are skewed in anyway from one another, a voltage differential for the CAN signal is created which results in radiated emissions. When the CAN signals, CANH and CANL, are perfectly symmetric, however, a minimal amount of radiated emissions exists.
In order to obtain high speed and symmetry, it is desirable to use low voltage, matched components. However, in this configuration, these low voltage components cannot withstand high voltage conditions due to gate oxide integrity issues and drain-to-source breakdown voltage limitations. High voltage components are not desirable for CAN applications due to their larger gate capacitances, and hence their slower operation. These high voltage components also incur a considerable silicon area penalty.
A known controller area network (CAN) transceiver includes an output driver as is shown in FIG. 1. Output driver 100 includes a CANH driver and a CANL driver. The CANH driver uses a pnp (or a PMOS) transistor MP1 as an active device, while the CANL driver uses an npn (or an NMOS) transistor MN1 as an active device. Voltages applied at nodes, VA and VB, turn those two transistors, MP1 and MN1, on and off, respectively. Thereby, the function of each driver is simply to turn transistors, MP1 and MN1, on and off. Typically, the external load that exists between the nodes CANH1 and CANL1 external to the chip is a 60 ohm termination. When transistors, MP1 and MN1, turned on, current flows through the 60 ohm termination load to ground. Thereby, a voltage is set up across nodes CANH1 and CANL1. Accordingly, the recessive and differential state is established. When transistors, MP1 and MN1, are turned off, there will be no differential between nodes CANH1 and CANL1. Once transistors, MP1 and MN1, turn off, no active control exits for the value of the voltage at nodes, CANH1 and CANL1. Thereby, nodes, CANH1 and CANL1, may settle to a value determined passively and in a time dictated by the time constant of the bus capacitance and internal resistance.
Moreover, radiated emissions may stem from non-idealities that exist when the CAN wires transition from the dominant state to the recessive state. Specifically, with reference to FIG. 2, the waveforms illustrate the voltage of the CAN wires ideally alternating from the recessive state to the dominant state and back to the recessive state. As shown, during the dominant state there is a differential in voltage between nodes, CANH1 and CANL1. During the recessive state, however, there is no differential between nodes, CANH1 and CANL1. FIG. 3 is an actual result of waveforms that display the voltage of the CAN wires ideally alternating from the recessive state to the dominant state and back to the recessive state. FIG. 4 displays a magnified version of the transition from the dominant state to the recessive state. As shown, there is some non-ideality at point B. This non-ideality is the third cause of radiated emissions in a CAN transceiver. The desired response is that both bus signals, CANH1 and CANL1, shift to a common-mode value at point A and remain at this common-mode value. During this transition, however, the PMOS transistor MP1 and the NMOS transistor MN2 in the output driver 100 are turning off. Once these transistors, MP1 and MN2, are off, there is presently no known method of regulating the voltage at these bus nodes, CANH1 and CANL1. Accordingly, these bus nodes, CANH1 and CANL1, transition to a voltage based upon existing conditions.
Thus, there exists a need for a differential CAN driver that includes circuitry that eliminates the non-ideality of the CAN wires during the transition from the dominant state to the recessive state. Furthermore, there exists a need for a CAN transceiver that is protected from high voltage transients. In addition, there exists a need for a CAN transceiver that can withstand shorts from the CAN wire to any power supply rail or ground.
The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.